This invention relates to a method for verifying thickness modulations, in particular watermarks, in or on sheet material, in particular documents of value or security documents.
In machine authenticity testing of documents of value or security documents such as bank notes, the verification of watermarks as an authenticity feature plays an important part. This is frequently done using optical methods by which a bank note to be checked is irradiated with light and the light transmitted through the bank note is detected with the aid of a one- or two-dimensional photodiode array. The obtained data are compared with data of a reference watermark, the bank note being classified as authentic or false in accordance with the degree of match.
However, optical methods are generally disadvantageous in that measurements can be easily disturbed by ambient light, and the sensitivity of the photodetectors customarily used as well as the strength of the light sources used are subject to temporal fluctuations that likewise falsify measurement. In addition, an imaging optic comprising lenses and/or diaphragms is generally required, which necessitates high adjustment effort during production and maintenance of corresponding measuring systems.
The invention is based on the problem of providing an improved method for verifying thickness modulations in or on sheet material that in particular reduces the influence of disturbances and the adjustment effort during production and maintenance of corresponding measuring systems.
This problem is solved by the invention in that the sheet material is subjected to sound, in particular ultrasound, the sound transmitted through the sheet material and/or reflected on the sheet material is measured at a plurality of places on the sheet material, and at least one value characteristic of the transmitted and/or reflected sound is generated for each of the places. Verification of thickness modulations is then done by comparing the generated characteristic values with predetermined reference values. The use of sound, in particular ultrasound, avoids the influence of disturbances, for example due to ambient light, and in addition reduces the adjustment effort.
In a preferred embodiment of the method it is provided that the places where the transmitted or reflected sound is measured are located on at least one track extending linearly on the sheet material. The distances between the individual places can be selected so as to obtain a certain spatial resolution of the watermark to be measured. In particular, the individual places can also touch or overlap each other so that a continuous track is measured on the sheet material.
The invention will now be explained in more detail with reference to figures, in which: